1. (Field of the Invention)
The present invention relates to a power supply circuit and, more particularly, to the power supply circuit for supplying a voltage via two DC-DC converters to a control circuit for controlling electric appliances.
2. (Description of the Related Art)
It is well known that depending on the type of electric appliances, the electric circuitry used therein requires the use of a plurality of power sources. By way of example, an air conditioner for air-conditioning a house room or the like space makes use of a control power source for supplying an electric power to a control circuit system including a controller that controls the air conditioner, an actuator power source for supplying an electric power to various driving mechanisms or actuators such as relays and stepping motors, and a drive power source for driving a fan motor. These power sources provide different direct current (DC) voltages appropriate to the different electric component parts to be fed. For example, the control power source provides a 5-volt DC voltage, the actuator power source provides a 12-volt DC voltageactuator power source, and the drive power source provides a variable DC voltage ranging of 0 to 40 volts.
FIG. 3 shows the prior art power supply circuit for a fan motor and control system circuits in the air conditioner. In this prior art power supply circuit, an AC voltage derived from a commercial AC power outlet 9 is rectified by a rectifier circuit 11, and the rectified DC voltage from the rectifier circuit 11 is then converted by a DC-DC converter 15 into a fixed output voltage V'.sub.1 and a variable output voltage V'.sub.2. Not only is the fixed output voltage V'.sub.1 (12 V) from the DC-DC converter 15 used as an actuator power source, but the fixed output voltage V'.sub.1 is, after having been converted into a predetermined voltage (5 V) in a regulator 17 which outputs a stabilized steady voltage, also used as a control power source. On the other hand, the variable output voltage V'.sub.2 from the DC-DC converter 15 is converted into a three-phase AC voltage by an inverter 19 and is then supplied to a fan motor 21. The magnitude of the variable output voltage V'.sub.2 is controlled according to a control duty signal derived from a controller 16.
The power loss W.sub.loss produced in the regulator 17 is expressed by the following equation: EQU W.sub.loss =I(V.sub.in -V.sub.out) (1)
where I is the consumption current of the control system circuits, i.e., the current consumed by the control system circuits, V.sub.in is the input voltage to the regulator 17 (i.e., the fixed output voltage V'.sub.1 of the DC-DC converter 15), and V.sub.out is the output voltage from the regulator 17.
Because the consumption current I of the control system circuits and the output voltage V.sub.out of the regulator 17 are both constant, the power loss W.sub.loss in the regulator 17 decreases with decrease of the fixed output voltage V'.sub.1 of the DC-DC converter 15, as seen from Equation (1).
During a standby state of the air conditioner, the fan motor 21 is kept halted with no variable output voltage V'.sub.2 consequently utilized, but the fixed output voltage V'.sub.1 is required to keep the control system circuits in operation. However, in this prior-art power supply circuit, during the standby state, a high voltage (12 V) equal to a voltage during the operation is outputted to the regulator 17 so that the latter can provide the voltage required by the control system circuits, while the regulator 17 requires a low voltage (e.g., 6 V) in order for it to produce a voltage (5 V) necessary to drive the control system circuits. Therefore, in the prior art circuit, a wasteful power loss arises in the regulator 17 as understood from Equation (1), resulting in an increased power consumption in the power supply circuit.